Sedimentary evolution under high-frequency sea-level oscillations of the lower Triassic carbonate-evaporite system in Xuanhan area, Sichuan Basin: Implications for the genesis of polyhalite minerals

Abstract

Polyhalite is a common potassium-bearing mineral of many ancient evaporitic systems, which can provide important ore resources for agricultural development. However, the genesis of polyhalite is still debated, especially for the origin of primary type. In order to provide novel perspectives and evidence for the genesis of polyhalite minerals, detailed sedimentological and sequence stratigraphic analyses, combined with sulfur isotope stratigraphy, were carried out using Lower Triassic core data in the Xuanhan area, northeastern Sichuan Basin. Fifteen microfacies grouped into six microfacies associations were identified. Furthermore, facies cycle types were recognized and the target intervals were subdivided into six fourth-order sequences and multiple parasequences within them. The result of fourth-order sequences was further validated by sulfur isotope chemostratigraphy. Sedimentary models in the study area and a formation model for primary polyhalite were established consequently. The coupled effects of paleoenvironmental conditions, tectonic activities, and provenance supply caused by sea-level fluctuations collectively led to the precipitation of primary polyhalite in the Sichuan Basin during the Early Triassic. Specifically, primary polyhalite occurs when K⁺- and Mg²⁺-enriched concentrated brine within halite saltern mixes with external influxes carrying Ca²⁺ and SO₄²⁻ ions, a process regulated by the dynamic fluctuations of multi-order sea-levels, especially the high-frequency ones. Moreover, the margins of the halite saltern and the gypsum saltern, as well as the spatial transition zone between them, created an ideal setting that can meet the above-mentioned geochemical conditions for the precipitation of polyhalite. These results contribute to the supplementation and refinement of the formation mechanisms of polyhalite minerals, especially for the primary ones.